This invention relates to optical disk data storage systems, and more particularly to a system and method for detecting and distinguishing coarse servo tracks from data tracks in a coarse servo system of an optical disk data storage system.
Optical data storage systems that utilize a disk to optically store information have been the object of extensive research. Like their counterpart magnetic disk units, these optical disk storage units must have a servo system which controls the positioning of a read/write head to provide direct access to a given track of data recorded on the rotating disk Further, once a desired track has been accessed, the servo system must cause the read/write head to accurately follow this track while it is being read or when data is initially written thereon to.
To date, most of the prior art optical storage systems have had one of three types of servo systems physical groove, external encoder or optical feedback. The simplest of the systems is the physical groove as is shown, for example, in U.S. Pat. Nos. 4,260,858 and 3,654,401. In such systems, the optical disks are provided with physical grooves either in a spiral or circular pattern, and an optical read/write head is provided with a stylus or other physical means of engaging the groove. There are several drawbacks associated with this type of system. First, wear is a significant factor. Typically, the disks are formed using dies or molds, which dies or molds are subject to wear during the manufacturing process, hence necessitating their replacement and creating tolerance problems in the formed disks. Physical contact with the head guide stylus during use also causes disk wear, introducing noise. Further, rapid access involving radial movement of the head is difficult to accomplish.
One known approach to overcome the problems associated with the physical groove systems is to dedicate an entire data disk surface to servo tracks. This approach has worked well in magnetic disk systems, where a plurality of magnetic disks are usually provided in a stacked disk pack with a common spindle. The use of one disk surface for servo tracks does not seriously detract from the data storage capacity of such a system. Optical disk systems, on the other hand, in order to be suitable for use in a commercial environment, desirably have only one disk on a spindle with at most two surfaces available for recording both the data and servo information. It is not feasible, therefore, to dedicate an entire optical disk surface to servo tracks without severely sacrificing data storage capacity.
While magnetic disk servo systems can be adapted for use with optical disks, this approach is also very inefficient. Data track density can be made much higher in optical recording systems than in magnetic disk systems. Optical systems are capable of recording in an extremely narrow data track approaching one micron in width. This allows an increase in track densities on the order of 15 times the densities used in state of the art magnetic disk systems. An extremely accurate and sensitive servo system must be used to position the read/write head over such a track.
The optical disk systems that have heretofore provided the highest capability have employed optical feedback for tracking. Changes in reflected or transmitted illumination received from the disk are monitored by appropriate equipment. Illumination changes indicate the occurrence and direction of an off-track condition. Appropriate circuitry senses the change and activates a galvanometer controlled mirror in the light path steering the light beam(s) in the appropriate direction to continue track following. Such tracking systems can be extremely accurate and responsive but have range limitations on the order of 100 microns. This limitation arises essentially from the optics through which the light travels between the mirror and the disk. Galvanometer mirror systems allow rapid random access within this range but the optical head must be moved across the disk to obtain access to wider areas. Modern data storage applications require fairly rapid access to any data storage area on the disk and thus require accurate track accessing over a range of many centimeters and accurate track following upon access.
Galvanometer type servo positioning systems typically access other tracks in one of three ways:
(1) after accessing a first track (the starting point), successively adjacent tracks are accessed, and identified, one at a time, until the desired track is reached,
(2) after identifying the track presently accessed (the starting track), and the track to be accessed (the target track), a determination is made as to the number of tracks n between the starting track and the target track, and whereupon access is achieved by moving the head n tracks, as determined by counting the individual tracks between the starting and target tracks; locking onto each track successively as the count progresses; or
(3) after identifying the starting and target tracks as in method (2) above, a velocity servo is enabled which achieves access by forcing the galvanometer to follow a prescribed velocity profile that steers the optical beam to the vicinity of the target track, whereupon the track identification is read to verify that the desired target track has been reached.
Access method (1) above is extremely slow. Method (2), on the other hand, provides faster access and can be realized with relatively simple counting circuitry. Method (3) provides the fastest access, but also requires the most complex circuitry for its realization. All three methods however, are limited to the range of the galvanometer system, and do not, therefore, provide the needed rapid random access to all portions of the disk.
It is also a desirable feature in commercial optical disk systems to provide a removable/replaceable disk. This allows disks to be readily changed so that information recorded on different disks can be easily accessed. In a removable/replacement disk system, gross errors in alignment (up to several hundreds of microns) of the disk with respect to the head are unavoidable. Such alignment errors will typically exceed the 100 micron tracking radius of most galvanometer systems. It is therefore necessary to provide a servo system which will compensate for these gross errors and which will reliably position the read/write head with direct access over a large area of the optical disk.
Various systems have been developed to improve random access, or compensate for gross positional errors, or both. For example, U.S. Pat. No. 4,094,010 utilizes plurality of fixed read/write heads spanning an entire disk surface Each head is associated with a single servo track and a band of data tracks. While rapid access is assured by such a system, the plethora of tracking heads and ancillary equipment required greatly increases the cost and complexity of the system. The optical systems of U.S. Pat. Nos. 4,275,275, 4,160,270 and 4,282,598 each develop a coarse tracking error signal for use by a coarse positioning system to control head movement during tracking. The coarse track signal is developed from the tracking error signal generated by the galvanometer fine tracking system In U.S. Pat. No. 4,037,252 a coarse control signal is generated from the movement of the fine tracking galvanometer mirror itself as opposed to the signal developed from illumination data obtained from the disk. A significant drawback of these coarse positioning systems is that they do not decouple fine tracking errors from coarse tracking errors, thereby providing a less stable system. Moreover, these systems provide no improved direct random access capability.
Whatever the type of access and tracking system employed, some sort of detection means must be used to generate an error signal that can be used by the appropriate servo system to guide the positioning of the read/write head to a desired radial position with respect to the disk, and to maintain this desired position once reached. In the above-cited 438,133 application, a coarse/fine servo system is disclosed that achieves this purpose. Coarse servo tracks, selectively placed on the disk, allow the coarse servo system to access and track a relatively large band on the disk. The fine servo system is then used to access and track a desired data track within the band. More particularly, according to the teachings of the 438,133 application, by selectively placing spaced-apart coarse servo tracks on the disk, and then by illuminating through the read/write head an area of the disk large enough to always include a segment of one of these coarse servo tracks, the reflected radiant energy from the illuminated coarse servo track becomes a narrow strip of radiant energy that may be directed back through the read/write head to the surface of a detector array. The signal generated by the array can then be used as an error signal to indicate the location of the read/write head relative to a given coarse track. This error signal is used, in turn, by a coarse positioning servo system to place the read/write head at a desired location so as to provide the requisite coarse access and tracking capability.
The detector array disclosed in the above-cited 438,133 application represented the best mode of carrying out the invention disclosed therein at the time the invention was made. The 503,955 application discloses an improvement to this best mode, particularly with respect to the type of detector array that is used and the manner of processing and generating the error signal.
However, for the types of systems disclosed in the above-cited applications, a particular problem exists in that radiation incident to the array may be reflected from more than just the desired coarse servo track, e.g., from data tracks. These data tracks may or may not be present on the disk depending upon how "full" the disk is with respect to its data storage capacity. Some means is needed therefore to identify and distinguish the desired reflected radiation coming from the servo tracks from any undesired reflected radiation that may be present, such as that which comes from data tracks adjacent to servo tracks.
What is needed, therefore, is a detection system and corresponding storage disk that provides a means for distinguishing radiation that is reflected from only a desired coarse servo track, and not from adjacent data tracks.